Correlation between serum iron levels and pulmonary function: A cross-sectional analysis based on NHANES database 5319 cases

Pulmonary function, one of the main indicators of respiratory system assessment, is difficult to measure in specific cases. The study investigated the association between serum iron levels and pulmonary function. The cross-sectional study was conducted using data from 5319 participants from the 2010–2012 National Health and Nutrition Examination Survey. Forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), and forced expiratory flow from 25% to 75% of FVC were used as indicators of pulmonary function to analyze the relationship of serum iron and pulmonary function. Univariate and stratified analyses, multiple equation regression analysis, smoothed curve fitting analysis, and threshold effect analysis were performed to explore the relationship between pulmonary function and serum iron concentrations. Threshold effect analysis revealed a nonlinear relationship between serum iron levels and FVC, as well as FEV1, with inflection points observed at 8.1 (µmol/L) and 8.4 (µmol/L), respectively. When serum iron concentrations fell below the inflection point, there was no statistically significant relationship between serum iron and FVC (P = .065) or FEV1 (P = .095) (P > .005). However, when serum iron concentrations exceeded the inflection point, both FVC (β = 6.87; 95% confidence interval [CI] = 3.95, 9.79; P < .0001) and FEV1 (β = 7.09; 95% CI = 4.54, 9.64; P < .0001) exhibited a positive correlation with increasing serum iron levels. Additionally, forced expiratory flow from 25% to 75% of FVC (mL/s) demonstrated a positive association with serum iron (β = 6.72; 95% CI = 2.30, 11.13; P = .0029). Serum iron level was positively correlated with pulmonary function within a certain range of serum iron concentration. Serum iron level may be a protective factor for pulmonary function.


Introduction
Pulmonary function is one of the key indicators for detecting respiratory diseases and is an important indicator for the functions of pulmonary ventilation and pulmonary defense. [1][3] However, in some populations, for example, coma, shock, mental disorders, and other patients who cannot cooperate, the above indicators can be difficult to obtain.Additionally, testing pulmonary function requires cooperation between the examiner and the patient, and the results can be influenced by a variety of technical and personal factors. [4]Blood gas analysis is an acute indicator that can only reflect the immediate situation of the patient.Blood gas analysis is not practical for assessing the overall pulmonary function of the patient. [5]erum iron is a trace element in the human body, which plays an important role in numerous physiological and pathological mechanisms of the human body. [6,7]Some studies have shown that serum iron is positively correlated with the cardiovascular system and renal function, [8,9] McKeever et al [10] demonstrated that higher serum iron was independently associated with higher FEV1.However, the association between serum iron and pulmonary function is uncertain.The National Health and Nutrition Examination Survey (NHANES) is a cross-sectional population-based database that annually surveys a nationally representative sample of people located in every state in the United States. [11]It includes diet-and health-related questions, demographic data, general screening data, laboratory data, etc.These data are used to determine disease risk factors and disease prevalence. [12,13]The wide sample of the database provides more reliable support for study.The aim of this study was to investigate whether serum iron levels affect lung function.The data used in this article are extracted from the NHANES database.The purpose of this study is to explore new predictors for the assessment of pulmonary function.

Study population
Data were downloaded by logging into the NHANES database website (Centers for Disease Control and Prevention, https:// www.cdc.gov/),selecting the data from the NHANES database for 2010-2012, then selecting the laboratory data option and study variable data, exporting the data in XPT format, and using R Studio for viewing.Retrospective analysis was then performed using R software and EmpowerStats(www.empowerstats.com).Data exclusion criteria included missing FVC, FEF 25%-75%, and FEV1 data, missing serum iron data, participants who were already pregnant and those missing from thoracoabdominal surgery, and individuals with stroke, heart attack, and cigarette date.

Variables
The exposure variables in this study were serum iron levels (µmol/L) and the outcome variable pulmonary function, with FVC, FEV1, and FEF 25%-75% selected for pulmonary function as the outcome assessment index.Three outcome variables were divided into 3 different groups and analyzed by R software and EmpowerStats.Subgroups were based on serum iron concentration tertile as low, medium, and high.Fourteen covariates were added for more accurate study results, including age (years), gender, race/Hispanic origin, education level, thoracic/ abdominal surgery, respiratory disease, standing height (cm) tertile, body mass index (kg/m 2 ), systolic blood pressure (mm Hg), diastolic blood pressure (mm Hg), alcohol, had food, diabetes, and cholesterol (mmol/L) tertile (Table 1).R software and EmpowerStats were used to analyze the relationship between changes in serum iron concentration and pulmonary function.The results were analyzed by observing β and P values.A standardized β value >0 is considered to be a coefficient of increase in pulmonary function and <0 is considered to be a coefficient of decay in pulmonary function.

Statistical analysis
The results were generated by Empower Statistics and R software.Univariate and multiple regression analyses were performed on the 3 serum iron concentration groups.Two model adjustments in the subgroup analyses were performed.The first adjustment variable included age, gender, and race and the second adjustment variables included age (years), gender, race/ Hispanic origin, education level, thoracic/abdominal surgery, respiratory disease, standing height (cm) tertile, body mass index (kg/m 2 ), systolic blood pressure (mm Hg), diastolic blood pressure (mm Hg), alcohol, had food, diabetes, and cholesterol (mmol/L) tertile (Table 3).Smoothed fitting curves were used to analyze the association between exposure variable and dependent variable (Fig. 1).

Selected participants' baseline characteristics
After the initial screening, data were obtained for 9412 participants.A total of 2365 participants with missing FVC, FEF 25%-75%, and FEV1 data, 1423 participants with missing serum iron data, and 54 participants who were already pregnant and those missing from thoracoabdominal surgery, and 251 participants with stroke, heart attack, and cigarette date were all excluded, leaving 5319 participants included in the study, in which 2668 were males and 2651 were females with a mean age of 39.88 and a standard deviation of 18.98 (Fig. 1 and Table 1 .The grouped data were derived using Kruskal-Wallis rank sum test.N = 1751 for the low concentration group, N = 1783 for the medium concentration group, and N = 1785 for the high concentration group, with no significant difference in the number of participants among the 3 groups (P < .0001).The average heights of the 3 groups were 156.41 cm, 167.04 cm, and 178.88 cm, respectively.The mean values of serum cholesterol in the 3 groups were 3.74 mmol/L, 4.74 mmol/L, and 6.04 mmol/L, respectively (P < .001).The results indicate that, among the covariates, age (years), gender, race/Hispanic origin, thoracic/abdominal surgery, respiratory disease, standing height (cm) tertile, body mass index (kg/m 2 ), systolic blood pressure (mm Hg), alcohol consumption, having food, diabetes, and cholesterol (mmol/L) tertile were all statistically significant (P < .05).In contrast, education level and diastolic blood pressure did not show statistical significance (P > .05).

Analysis of the association between serum iron and pulmonary function on a univariate basis
Serum iron was positively correlated with pulmonary function.Univariate analysis showed that FVC, FEV1, and FEF 25%-75% were all P < .0001.In the univariate analysis, age (years), gender, race/Hispanic origin, education level, thoracic/ abdominal surgery, respiratory disease, standing height (cm) tertile, body mass index (kg/m 2 ), systolic blood pressure (mm Hg), alcohol, had food, diabetes, and cholesterol (mmol/L) tertile were statistically significant (P < .05).On the other hand, diastolic blood pressure did not exhibit statistical significance (P > .05).The results revealed that individuals over the age of 60 faced a higher risk compared with those aged 40 to 60, and serum iron concentrations were predominantly higher in men (Table 2).The β value increased with higher education levels (P < .05).Risk factors were elevated with higher body mass index and serum cholesterol levels.The β values for the group without diabetes, no history of thoracic and abdominal surgery, and no respiratory disease were significantly higher than that of the control group (P < .0001).Furthermore, patients with no history of alcohol consumption exhibited higher risk factors (P < .0001).Serum iron exhibited a positive correlation with www.md-journal.com

Serum iron and pulmonary function analyzed by multiple regression equations
Further analysis of the association between serum iron and pulmonary function by multiple regression revealed that serum iron was positively associated with FVC, FEV1, and FEF 25%-75%.Two adjustments were performed for covariates after grouping serum iron concentrations.In the high serum iron concentration group, the P values for FVC, FEV1, and FEF 25%-75% were statistically significant both before and after variable adjustment.Moreover, with the low concentration group as the reference baseline, the high concentration group showed significantly higher β values for all 3 indexes compared with the medium concentration group.The β values increased with the increase of iron level.The β values, CIs, and P values of all covariates before and after adjustment are shown in Table 3. Table S1, Supplemental Digital Content, http://links.lww.com/MD/L24shows multiple regression equation analysis for all subgroup variables (Table S1, Supplemental Digital Content, http://links.lww.com/MD/L24, the multiple regression analysis of all subgroup variables).

Statistical analysis of smooth curve fitting, threshold effect, and saturation effect between serum iron levels and pulmonary function
Threshold effect analysis revealed a nonlinear relationship between serum iron levels and FVC, as well as FEV1, with    4. Smoothing curve fitting was used to assess whether serum iron levels were related to pulmonary function.The results indicated that serum iron exhibited a nonlinear correlation with FEV1 and FVC.However, there was a linear correlation between serum iron and FEF 25%-75%.Beyond the inflection point, FEV1 and FVC exhibited a significant increase with rising serum iron concentration until reaching a plateau.FEF 25%-75% displayed a linear positive correlation with increasing serum iron concentration (Fig. 2).
In the smooth curve analysis depicting the relationship between serum iron and FVC, most covariables demonstrated that FVC increased as serum iron concentration increased.The curve fitting for individuals with body mass index >25 and diastolic blood pressure > 90 exhibited substantial fluctuations, demonstrating an S-shaped curve (Figure S1, Supplemental Digital Content, http://links.lww.com/MD/L25, the smooth fitting curves of all covariables with FVC).In the smoothed curve analysis of the relationship between serum iron and FEV1, most of the covariates showed a concomitant increase in FEV1 with increasing serum iron.The curve fitting for individuals with diastolic blood pressure > 90 exhibited substantial fluctuations, demonstrating an S-shaped curve (Figure S2, Supplemental Digital Content, http://links.lww.com/MD/L26, the smooth fitting curves of all covariables with FEV1).In a smoothed curve analysis of the relationship between serum iron and FEF 25%-75%.The curve fitting for all covariables displayed an upward trend (Figure S3, Supplemental Digital Content, http://links.lww.com/MD/L27, the smooth fitting curves of all covariables with FEF 25%-75%).

Discussion
[16][17][18][19][20] Pulmonary function, as the main assessment indicator of the respiratory system, plays a crucial role in the diagnosis and treatment of diseases; however, there are some contraindications to pulmonary function tests. [21,22]urrently, the limitations of pulmonary function tests have not been addressed by the introduction of better tests. [21]This study found that serum iron concentrations may be a good indicator for assessing pulmonary function.
The regulation between pulmonary function and serum iron concentrations was analyzed by screening the NHANES database, a population-based cross-sectional survey that surveys a nationally representative sample each year.This study obtained 5319 participants eligible for our study. [11]Many researchers have found many meaningful studies through this database.The study yielded interesting and noteworthy findings from the analysis of large, rigorous, and authentic data from the NHANES database.
Serum iron, an important element in the body, plays a crucial role in maintaining homeostatic balance in the body.Serum iron is essential for all cell division and is required for DNA biosynthesis.Iron is required for daily metabolism and biosynthesis, mainly for the biosynthesis of hemoglobin to meet the daily requirement of red blood cells. [23]Serum iron is involved in the synthesis of many proteins and plays an important role in proteins related to oxygen metabolism.The oxygen-carrying capacity of hemoglobin directly affects the pulmonary function. [24]ron deficiency has been shown to be associated with reduced exercise capacity, peak oxygen consumption, and 6-minute walk test values in humans.27] New study by Ueda et al [8] showed that serum iron concentration can be a predictor of adverse outcomes in patients with acute decompensated heart failure.Del Greco et al [28] study found positive effects of iron on kidney function in the common population.Serum iron may be in improving cardiac function, renal function, at the same time, improve the pulmonary function.
29][30] FEV1, FVC, and FEF 25%-75% were selected as the outcome variables of the study because these 3 indicators are not only commonly used in clinical practice but also reflect the pulmonary function more accurately. [31,32]Serum iron concentration was found to be positively correlated with pulmonary function, which may be related to age, gender, race, and other factors.After controlling for the effects of the relevant single factors in the multiple regression analysis, the results still revealed a linear positive association between serum iron and pulmonary function.The results of the smooth fitting curve also confirm the positive associations between serum iron concentration and pulmonary function.
It was found that the bone morphogenetic protein pathway regulates serum iron concentrations by regulating hepcidin protein, a key protein in the regulation of iron metabolism. [23,33,34]When hepcidin is elevated, it leads to decreased iron uptake and extracellular transport and decreased serum iron concentration, resulting in increased levels of oxidative stress and impaired function of the vascular endothelium.Iron is a strong redox agent and has an important role in protecting vascular endothelial stability.Hypoxia and inflammatory factors induce upregulation of hepcidin.Iron deficiency may be involved in hypoxic pulmonary hypertensive vascular injury by upregulating pulmonary artery endothelial cell levels and promoting apoptosis of endothelial cells and proliferation of lung artery smooth muscle cells, and further upregulation of hepcidin levels leads to a vicious cycle of iron deficiency. [35]It is believed that serum iron concentration affects pulmonary function.Existing research shows that the iron in the pulmonary vascular function perception of oxygen plays an essential role.Decker et al [36] demonstrated that the abnormal level of serum iron deficiency is strongly associated with the clinical severity of pulmonary hypertension; Livesey et al [37] showed that insufficient or lost serum iron is associated with deep vein thrombosis and pulmonary embolism.Viethen et al [38] treated patients with iron deficiency symptoms in pulmonary hypertension with intravenous iron therapy, resulting in a marked improvement in quality of life and the 6-minute walk test compared with the untreated group.Iron deficiency in anemic COPD patients is correlated with a moderate increase in systolic pulmonary artery pressure and a limitation of diffusion capability; in Eisenmenger patients, iron deficiency affects heart function as well as pulmonary function. [29]Iron supplementation has been shown to improve symptoms and quality of life in patients with pulmonary hypertension and heart failure. [8]Vaugier et al [9] found that elevated serum iron levels in patients may improve the prognosis of renal transplantation and improve renal function and that the improvement in renal function, which alters the intrinsic mechanisms of circulating blood flow, also affects liver function and pulmonary function.None of these studies indicate whether serum iron concentration directly affects pulmonary function.A study by Sato et al [39] demonstrated that elevated serum iron levels can counteract the harmful effects of smoking on pulmonary function in Japanese men.This suggests that serum iron could have a significant impact on lung function.However, only male smokers participated in the study and there was no comparison with female or nonsmokers.A study by Zhang et al investigated whether iron supplementation could exacerbate smoking-related pulmonary iron accumulation and increase the risk of infection The authors also suggested that iron may be a new goal in COPD treatment. [40]McKeever et al [10] demonstrated that trace iron was independently associated with higher FEV1, while Ghio et al [41] found that decreases in serum iron levels were independently associated with decreases in FVC and FEV1.The results of this study provide evidence that serum iron levels may be protective factors for pulmonary function and, therefore, serum iron levels may be a simple and accurate blood test to assess pulmonary function.The present study included 5319 participants.This study has the advantage of large sample size, wide study population, and rigorous statistical methods.The results of confirmed that serum iron concentration was positively correlated with pulmonary function and that serum iron is a favorable factor for pulmonary function.Regulation of pulmonary function by serum iron levels may be a possibility in the future and provide guidelines for predicting pulmonary function in specific populations.Currently, however, this study can be used as a guide to improve pulmonary function in pre-and postoperative patients.Serum iron not only affects renal function and cardiovascular system but also significantly affects pulmonary function.To our knowledge, this is the first study to demonstrate a positive correlation between serum iron levels and pulmonary function.
Simultaneously, the study also demonstrated that the combination of alcohol and dietary factors can enhance lung function to some extent.Furthermore, serum iron concentration is advantageous for improving lung function in diabetic patients.
This study has some limitations, first, the sample itself has limitations; the participants limited to the American population, and results may be different for other regions.Second, although the study controlled for relevant complex factor variables, other factors cannot be excluded.Third, the molecular mechanisms underlying the effects of serum iron on pulmonary function are unclear and require further basic research.Fourth, this study did not consider whether serum iron in patients with impaired pulmonary function.Finally, the present study did not consider whether serum iron affects patients with impaired pulmonary function.

Conclusions
Serum iron levels are protective factors for pulmonary function.Serum iron levels may provide a simple and accurate blood test to assess pulmonary function, providing a guide to predict pulmonary function in specific populations.Appropriate serum iron supplementation may improve pulmonary function.

Figure 2 .
Figure 2. The association between serum iron and pulmonary function.The smooth curve fit between the variables is indicated by the red line.The fit's 95% confidence interval is represented by the blue bar.(A)Scatter plot of curve fitting of serum iron levels and FEF 25%-75%.(B) Scatter plot of curve fitting of serum iron levels and FEV1.(C) Scatter plot of curve fitting of serum iron levels and FVC.Adjusted for age(years), gender, race/Hispanic origin, education level, thoracic/abdominal surgery, respiratory disease, standing height (cm) tertile, body mass index (kg/m 2 ), systolic blood pressure (mm Hg), diastolic blood pressure (mm Hg), alcohol, had food, diabetes, cholesterol (mmol/L) tertile.FEF 25%-75% = forced expiratory flow from 25% to 75% of FVC, FEV1 = forced expiratory volume in 1 second, FVC = forced vital capacity.

Table 2
Univariate analysis of serum iron and pulmonary function.
and inflammation in these patients.Zhang et al concluded that further research should investigate how iron affects lung function.